Water Research 37 (2003) 2140–2148
Effects of chromium (VI) addition on the
activated sludge process
Athanasios S. Stasinakisa,*, Nikolaos S. Thomaidisa, Daniel Mamaisb,
Evangelia C. Papanikolaoua, Angeliki Tsakona, Themistokles D. Lekkasa
a
Water and Air Quality Laboratory, Department of Environmental Studies, University of the Aegean, Theofrastou and Alkaiou Str.,
Mytilene 81 100, Greece
b
Faculty of Civil Engineering, Department of Water Resources, National Technical University of Athens, 5 Iroon Polytechniou Str.,
Zografou, Athens 15773, Greece
Received 4 March 2002; accepted 25 November 2002
Abstract
The effect of hexavalent chromium, Cr(VI), addition on various operating parameters of activated sludge process was
evaluated. To accomplish this, two parallel lab-scale continuous-flow activated sludge plants were operated. One was
used as a control plant, while the other received Cr(VI) concentrations equal to 0.5, 1, 3 and 5 mg l1. Cr(VI)
concentrations of 0.5 mg l1 caused significant inhibition of the nitrification process (up to 74% decrease in ammonia
removal efficiency). On the contrary, the effect of Cr(VI) on organic substrate removal was minor for concentrations up
to 5 mg l1, indicating that heterotrophic microorganisms are less sensitive to Cr(VI) than nitrifiers. Activated sludge
floc size and structure characterization showed that Cr(VI) concentrations higher than 1 mg l1 reduced the filaments
abundance, causing the appearance of pin-point flocs and free-dispersed bacteria. Additionally, the variability of
protozoa and rotifers was reduced. As a result of disperse growth, effluent quality deteriorated, since significant
amounts of suspended solids escaped with the effluent. Termination of Cr(VI) addition led to a partial recovery of the
nitrification process (up to 57% recovery). Similar recovery signs were not observed for activated sludge floc size and
structure. Finally, shock loading to the control plant with 5 mg l1 Cr(VI) for 2 days resulted in a significant inhibition
of the nitrification process and a reduction in filamentous microorganisms abundance.
r 2002 Elsevier Science Ltd. All rights reserved.
Keywords: Toxicity; Wastewater; Nitrification; COD removal; Sludge settling
1. Introduction
The increasing trend towards combining industrial
and municipal wastewater for treatment in sewage
plants increases the possibility of contamination of the
influent by metal ions. Although the mechanisms by
which heavy metals affect the biological treatment
processes are not well defined, it is well documented
that relatively low concentrations of various heavy
*Corresponding author. Tel.:+30-22510-36225; fax: +3022510-36226.
E-mail address: [email protected] (A.S. Stasinakis).
metals may stimulate the biological systems, while
increased concentrations may partially reduce system
performance [1–3].
Chromium is usually encountered in the environment
at oxidation states of (III) and (VI). It is released by
effluent discharge from steelworks, chromium electroplating, leather tanning and chemical manufacturing.
Each of these oxidation states has very different
biological and toxicological properties. Cr(III) accumulates in the cell membrane and is considered to be less
toxic. On the contrary, Cr(VI) is transported into the
cells, where it is reduced to the trivalent form and reacts
with intracellular material [4].
0043-1354/03/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0043-1354(02)00623-1
A.S. Stasinakis et al. / Water Research 37 (2003) 2140–2148
The effect of Cr(VI) on substrate removal, respiration
activity and bacterial growth in activated sludge systems
has been studied previously, but the results were
controversial, in most cases. In particular, early works
by Barth et al. [5] and Moore et al. [6] on substrate
removal supported that aerobic biological treatment
processes could tolerate, without significant loss in
treatment efficiency, Cr(VI) concentrations in the range
of 10–50 mg l1. Moreover, Moore et al. [6] showed that
at a concentration of 5 mg l1 Cr(VI), the unit
performed better than the control reactor. However,
Lamb and Tollefson [7] reported that activated sludge
shock loading of 5 mg l1 CrO2
reduced organic
4
substrate removal by 50%. Vankova et al. [8] studied
the effect of Cr(VI) on biomass respiration activity
and reported that the 1-h EC50 value was in the range of
40–90 mg l1, whereas Madoni et al. [9] reported that 1-h
exposure of activated sludge to a concentration of
83 mg l1 dissolved Cr(VI) reduced the specific oxygen
uptake rate (SOUR) only by 21.5%. Finally, Gokcay
and Yetis [10] and Yetis et al. [11] showed that activated
sludge was stimulated in the presence of Cr(VI). They
observed an approximately two times increase in
maximum specific growth rate, mm values and stimulatory effects on biomass yield in the presence of 25 mg l1
Cr(VI). On the contrary, Mazierski [12] and Stasinakis
et al. [3] observed a significant inhibition of heterotrophic growth in the presence of 10 mg l1 Cr(VI).
Though it is reported that nitrifying organisms may be
much more sensitive to heavy metals than heterotrophic
organisms [5,13], only the effect of Cr(III) on the
nitrification process has been investigated in continuousflow reactors [14]. Moreover, Cr(VI) effects on the size
and morphology of activated sludge flocs, the settling
capacity and the presence of higher microorganisms
have not been investigated at all.
From the above-mentioned literature review, it is
evident that Cr(VI) toxicity on the processes and
microbiology of activated sludge remains to be clarified.
Thus, the purpose of this study was to investigate the
effect of Cr(VI) continuous and shock loading on the
removal of organic loading and on the nitrification
process. Moreover, the Cr(VI) effect on various
secondary operating parameters of activated sludge
process, such as the size distribution of activated sludge
flocs, the settling capacity, the abundance of filamentous
microorganisms and the presence of protozoa and
rotifers, was investigated.
2. Materials and methods
2.1. Activated sludge pilot plants
Two parallel continuous-flow activated sludge plants
were operated during this study. One system was used as
2141
a control, receiving only synthetic wastewater (plant A),
whereas the other (plant B) received Cr(VI) in order to
investigate Cr(VI) toxicity.
The aerobic reactors of both systems were cylindrical,
continuously fed, plastic tanks with a liquid volume of
6 l. Aeration and efficient mixing were provided using
porous ceramic diffusers. The temperature within the
activated sludge units was kept at 20711C and the
dissolved oxygen (DO) was maintained above 4.0 mg l1.
To achieve a sludge age (yc ) of 8 days, the appropriate
amount of mixed liquor suspended solids was wasted
directly from the aerobic reactors, on a daily basis.
2.2. Experimental procedure
Activated sludge from a nitrifying municipal wastewater treatment plant that received no industrial wastewater (Plomari, Lesvos) was used to seed the reactors.
During the first 2 weeks, both systems operated on
synthetic wastewater (Table 1), devoid of Cr(VI) for
biomass acclimatization. A potassium dichromate solution (K2Cr2O7, Merck) was added to the synthetic
wastewater to provide a constant concentration of
0.5 mg l1 Cr(VI) to the influents of the experimental
activated sludge system (plant B) from the 15th day and
for a period equal to 3 yc (24 days). Cr(VI) was further
increased to 1, 3, 5 mg l1 at the 39th, 61st and 85th day,
respectively. After completion of the last experimental
period, no Cr(VI) was added to plant B for a period of
12 days in order to investigate the recovery capability of
the system. Additionally, 5 mg l1 Cr(VI) were added for
a 48-h period to the control system (plant A), to
investigate Cr(VI) effect on a non-acclimatized activated
sludge system.
2.3. Analytical methods
Analyses of influent and effluent COD (dissolved,
CODdis and total, CODtot), suspended solids, and mixed
liquor suspended solids were performed every 2 days,
according to Standard Methods [15]. Suspended solids
samples were obtained after filtration through 0.45 mm
pore size glass fiber filters (Whatman GF/C). Ammonium, nitrate and phosphate concentrations were
Table 1
Composition of wastewater solution fed to the continuous-flow
pilot plants
Constituent
Concentration (mg l1)
CH3COOH
NH4Cl
K2HPO4
KH2PO4
Other micronutrients
330
100
250
50
—a
a
Micronutrients were supplied using tap water as diluent.
2142
A.S. Stasinakis et al. / Water Research 37 (2003) 2140–2148
determined on filtered influent and effluent samples
(Millipore membrane filters, 0.45 mm pore size). Ammonium ions were determined by an acidimetric method,
while nitrate and phosphate were determined by ion
chromatography. Dissolved oxygen (DO), temperature
and pH values were measured daily in both systems. A
WTW Oxi 96 portable instrument was used for DO and
temperature measurements, while pH was determined
using Crison micropH2001.
Sludge volume index (SVI) and settling velocity were
measured in a 1-l graduated cylinder according to
Standard Methods. Activated sludge flocs size distribution was determined using a Mastersizer E instrument
(Malvern). This instrument uses light scattering and
data are given as frequency by volume [16]. In order to
investigate the morphology of activated sludge flocs, the
existence of higher microorganisms and the abundance
of filamentous microorganisms, a Leica phase contrast
microscope was used. For the identification of filamentous microorganisms, activated sludge samples were
analyzed every week according to Jenkins et al. [17].
3. Results and discussion
3.1. Start-up procedure
At the start of the experiment and for the first 14 days,
both systems were operated devoid of Cr(VI) to
acclimatize biomass to the synthetic substrate. Similar
values of various parameters were obtained in both pilot
plants. In particular, the reduction of dissolved COD
was greater than 96%, more than 93% of the
ammonium nitrogen (NH4/N) was nitrified, the SVI
values ranged between 60 and 65 ml gr1, the size
distribution of the flocs in the aeration tanks was
similar, the abundance of filamentous microorganisms
was between 1.5 and 2 [17] and the same genus of
microfauna were observed. To investigate the effect of
Cr(VI) continuous loading on the activated sludge
process, at the 15th day, Cr(VI) loading to the
experimental system (plant B) commenced at a level of
0.5 mg l1. At the 39th day, Cr(VI) addition was
increased to 1 mg l1, at the 61st day to 3 mg l1 and at
the 85th day to 5 mg l1.
3.2. Effect of Cr(VI) on the nitrification
During the whole period of the experiment, almost all
of the NH4/N was nitrified in the control system
receiving no Cr(VI) (plant A, mean ammonia removal
96.7%, with a standard deviation 1.6%). On the
contrary, Cr(VI) addition of 0.5 mg l1 to the experimental system (plant B) affected significantly the
nitrification process, as indicated by a gradual reduction
in effluent nitrate and increase in ammonium concentrations and pH. For instance, ammonia removal in plant B
reduced to 74% and 45% at the 25th and 37th day,
respectively (Fig. 1). Further increase of Cr(VI) concentration to 1 and 3 mg l1 led to a greater inhibition of the
nitrification process. Finally, a loading of 5 mg l1
Cr(VI) caused a further decrease in ammonia removal
rate to less than 30% (Fig. 1). Once chromium loading
to plant B was terminated (Day 109), the system began
to recover. After 12 days of ceasing chromium addition,
ammonia removal increased from 30% (Day 109) to
57% (Day 120), indicating that the system recovery from
Cr(VI) toxicity is a rather slow process.
In order to investigate acute Cr(VI) toxicity, the
control system, which never received any chromium
Fig. 1. Ammonia removal efficiency of activated sludge plants A (control) and B (experimental), during the experimental period.
A.S. Stasinakis et al. / Water Research 37 (2003) 2140–2148
addition, was shock loaded with 5 mg l1 Cr(VI) for a
48-h period (Days 117, 118). The addition of Cr(VI)
inhibited nitrification process, reducing ammonia removal to less than 45% (Fig. 1).
Inhibition of nitrification process was also evaluated
by determining nitrification rates, normalized at 201C,
during the various operating periods. Mean nitrification
rate values at 201C, qN20 ; were determined using Eqs. (1)
and (2), assuming that the nitrogen required for cell
growth (Nsynthesis ) is obtained as NH4/N and is
approximately 15% of the total influent nitrogen
(NH4/Ninfluent ) [18]:
ðNH4 =Nnitrified 1000Þ
;
V MLVSS
ð1Þ
qNT ¼ qN20 exp½0:116ðT 20Þ;
ð2Þ
qNT ¼
where qNT is the nitrification rate at T1C
(mgN gVSS1 h1), qN20 the nitrification rate at 201C
(mgN gVSS1 h1),
NH4/Nnitrified=(NH4/Ninfluent)–
(NH4/Neffluent)–Nsynthesis (kg d1), NH4/Ninfluent the total
influent nitrogen (kg d1), NH4/Neffluent the total effluent
nitrogen (kg d1), Nsynthesis the nitrogen required for cell
growth (kg d1), V the aerobic reactor volume (l),
MLVSS the concentration of volatile suspended solids in
the aerobic reactor (g l1) and T temperature of the
experiments (1C).
As illustrated in Table 2, addition of 0.5 mg l1 of
Cr(VI) reduced mean qN20 by 27%, from
5.83 mgN gVSS1 h1 in the control system (plant A)
to 4.24 mgN gVSS1 h1 in the experimental system
(plant B).
The removal of ammonium is of principal importance
during the activated sludge process for various reasons
including, the increase in chlorine demand during
wastewater disinfection, the ammonium contribution
to eutrophication and ammonia toxicity to fish and
other aquatic organisms. Although chromium is often
detected in activated sludge treatment plants [19,20], so
far only the effect of Cr(III) on the nitrification process
has been studied in a continuous flow system. Harper
2143
et al. [14] reported that continuous loading with
concentrations of Cr(III) up to 20 mg l1 did not reduce
the ammonia removal, while temporary signs of process
failure were observed for concentrations equal to
25 mg l1. However, the ammonia removal was fully
recovered in a period of 7 days, while shock loading with
40 mgl1 of Cr(III), for a 48-h period, did not cause any
upset to the system [14].
Comparing the results of our study with the data from
Harper et al. [14], it can be concluded that Cr(VI) is
much more toxic that Cr(III) to the nitrifying microorganisms. Although there are no previous data in the
literature comparing the effect of chromium species on
the nitrifiers, Cr(VI) is considered more toxic than
Cr(III) [21]. Studies on Cr(VI) toxicity have not
progressed enough to allow a deep understanding of
toxicity in vivo, especially for bacteria [22]. However, it
is generally assumed that its toxicity is associated with
its ability to penetrate the cell membrane. Within the
cell, Cr(VI) is reduced, via Cr(V) formation, to Cr(III),
which forms substitution-inert complexes with a number
of cell components [23].
3.3. Effect of Cr(VI) on CODdis removal
The effect of Cr(VI) on CODdis removal efficiency is
shown in Table 3. CODdis removal efficiency was
calculated based on the comparison between CODdis
concentrations at the inlet and outlet of each system. A
slight reduction was observed in the Cr(VI)-fed system
as compared to the control reactor for concentrations
equal to and higher than 1 mg l1. The difference in
CODdis removal between the two plants was not
statistically significant (at a 95% confidence interval,
using t-test) for Cr(VI) concentrations in the range from
0 to 1 mg l1, while it became statistically different, at
Cr(VI) concentrations equal or greater than 3 mg l1.
However, even at the highest Cr(VI) concentration
(5 mg l1), the difference in CODdis removal between the
two systems never exceeded 10%. Finally, 5 mg l1 of
Table 2
Mean nitrification rate, qN20, during the experiment
Experimental phases
Activated sludge plant A: control—no
Cr(VI) addition through phases A–D
qN20 (mgN gVSS1 h1) (mean7standard
deviation)
Activated sludge plant B: experimental
plant
qN20 (mgN gVSS1 h1) (mean7standard
deviation)
Acclimatization
Phase A: 0.5 mg l1 Cr(VI)
Phase B: 1 mg l1 Cr(VI)
Phase C: 3 mg l1 Cr(VI)
Phase D: 5 mg l1 Cr(VI)
Recovery
Shock loading
5.8070.91 (n ¼ 7)
5.8370.83 (n ¼ 12)
5.4870.88 (n ¼ 11)
5.5570.49 (n ¼ 12)
5.370.43 (n ¼ 12)
5.370.43 (n ¼ 5)
1.971.0 (n ¼ 2)
5.6770.82 (n ¼ 7)
4.2470.82 (n ¼ 12)
2.8471.06 (n ¼ 11)
2.5270.29 (n ¼ 12)
1.4370.32 (n ¼ 12)
1.7470.2 (n ¼ 7)
—
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A.S. Stasinakis et al. / Water Research 37 (2003) 2140–2148
Table 3
Mean CODdis removal efficiency (%) during the experiment
Experimental phases
Activated sludge plant A: control—no
Cr(IV) addition through phases A–D
CODdis removal (%) (mean7standard
deviation)
Acclimatization
Phase A: 0.5 mg l1 Cr(VI)
Phase B: 1 mg l1 Cr(VI)
Phase C: 3 mg l1 Cr(VI)
Phase D: 5 mg l1 Cr(VI)
Recovery
Shock loading
98.072.8
94.575.1
90.573.4
89.675.0
89.275.8
90.975.0
90.574.3
(n ¼ 7)
(n ¼ 12)
(n ¼ 11)
(n ¼ 12)
(n ¼ 12)
(n ¼ 5)
(n ¼ 2)
Cr(VI) shock loading for a period of 2 days did not
affect CODdis removal obtained in the control system
(plant A).
According to the literature, the critical Cr(VI)
concentration that affects substrate removal, ranges
from 5 to 50 mg l1. This discrepancy in experimental
results could be explained by the dependency of
metal toxicity on various parameters, such as the
operational conditions of activated sludge process,
the acclimatization of the biomass to the toxic
compound and the chemical and microbial speciation
[24–26,3,27].
The results reported in this study are in accordance
with a previous study referring to the effect of Cr(VI)
on the bacterial kinetics of heterotrophic biomass,
where similar experimental conditions were used
(substrate, sludge age, acclimatized biomass). In that
study, 1 mg l1 Cr(VI) caused a slight decrease in the mm
and biomass yield (YH ) values of heterotrophic
microorganisms, while significant inhibition was observed at Cr(VI) concentrations equal or greater than
10 mg l1 [3].
Comparing the effects of Cr(VI) on the nitrification
process and on the CODdis removal efficiency, obtained
from this study, we could support that nitrifying
organisms seem to be much more sensitive to Cr(VI)
than heterotrophic microorganisms. Similar observations for the sensitivity of nitrifiers to heavy metals have
been reported previously for nickel [28]. The higher
sensitivity of nitrifiers is mostly attributed to the fact
that nitrification is performed exclusively by two species
of chemo-autotrophic nitrifying bacteria, Nitrosomonas
spp. and Nitrobacter spp., whereas COD removal is
performed by a great variety of heterotrophic microorganisms. As nitrifiers have slow growth rates and are
only present in the mixed liquor in very small numbers,
compared with heterotrophic bacteria, even a small
reduction in their growth rate, caused by the presence of
heavy metals, may result in their washout and in
nitrification inhibition [29].
Activated sludge plant B: experimental
plant
CODdis removal (%) (mean7standard
deviation)
96.175.7
93.776.3
87.675.2
85.175.1
82.378.9
82.975.9
—
(n ¼ 7)
(n ¼ 12)
(n ¼ 11)
(n ¼ 12)
(n ¼ 12)
(n ¼ 7)
3.4. Effect of Cr(VI) on activated sludge floc
morphology and floc size distribution
During the first 40 days of the experiment, activated
sludge flocs size distribution and structure were similar
in both plants (Fig. 2a), characterized by the little filament effect on flocs structure (filament abundance: 1.5–2)
and the sufficient settling of suspended solids (SVI: 60–
65 ml g1). Following start up, the continual use of
readily biodegradable substrate (acetic acid) as the sole
source of carbon, led to a gradual increase of filament
abundance and caused activated sludge bulking in the
control system (plant A). As a result, sludge settling
gradually deteriorated, SVI values increased to 200, 490
and 835 ml g1 at the 61st, 85th and 109th day,
respectively (Fig. 3), whereas filamentous bacteria abundance increased to 4.5–5.5. Microscopic investigation
revealed that the dominating type was Sphaerotilus
natans and the secondary filaments were Type 021N.
These filamentous microorganisms appear to be favored
by soluble, readily metabolizable substrate and are often
observed in laboratory-activated sludge units [30,17].
On the contrary, bulking was not observed in the
experimental activated sludge system (plant B). During
the first 40 days of the experiments, SVI values and
filaments abundance for the two systems were similar
(Fig. 3). Increase of Cr(VI) loading to 3 mg l1 reduced
the concentration of filamentous microorganisms (filament abundance: 0.5) and caused a rapid decrease in the
size of activated sludge flocs in the experimental system
plant B (Fig. 2b). The sludge in plant B was characterized by the existence of fairly dense and more regularedged flocs, while the turbidity and the concentration of
suspended solids at the effluent increased (Fig. 4).
Activated sludge flocs settled rapidly producing pinpoint flocs with a low SVI (o30 ml g1) (Fig. 3) and a
highly turbid effluent. Due to the escape of suspended
solids with the final effluent, effluent COD values
increased at Cr(VI) concentrations equal or higher than
3 mg l1. Further increase of Cr(VI) to 5 mg l1 led to a
A.S. Stasinakis et al. / Water Research 37 (2003) 2140–2148
2145
Fig. 2. Floc size distributions: (a) acclimatization period; (b) in the presence of 3 mg l1 of Cr(VI) in activated sludge plant B; (c) in the
presence of 5 mg l1 of Cr(VI) in activated sludge plant B.
greater reduction of activated sludge floc size (Fig. 2c), a
complete absence of filamentous microorganisms and a
significant increase in the free-dispersed bacteria population in the mixed liquor.
The termination of Cr(VI) addition was not followed
by system recovery, as the effluent suspended solids
concentration from plant B remained high (Fig. 4).
Similar response was obtained in the control system
(plant A) when a shock loading of 5 mg l1 Cr(VI) was
applied for 2 days. Filaments abundance was reduced
and SVI values decreased from 500 to 85 ml gr1 (Fig. 4,
120th day).
It is widely accepted that the solids removal efficiency
in a clarifier depends directly on the size and structure of
flocs [31–33]. Various process parameters, such as solids
retention time [34], organic loading [35] and DO
concentration [16] have been reported to influence the
size distribution of the activated sludge flocs. Referring
to the effect of heavy metals on the size of activated
sludge flocs, similar observations, with this study, have
been reported for Hg, Cd, Zn and Cu [36,7].
Based on microscopic observations and determination
of flocs size distribution, it was observed that Cr(VI)
presence, initially affected the abundance of filamentous
2146
A.S. Stasinakis et al. / Water Research 37 (2003) 2140–2148
tain their integrity. It is possible that during experiment,
the addition of higher concentrations of Cr(VI) affected
the production of extracellular polymers, leading to the
entire break up of the flocs.
3.5. Effect on activated sludge micro fauna
Fig. 3. Comparison of SVI values in activated sludge plants A
(control) and B (experimental) during the experiment.
Fig. 4. Concentration of suspended solids at the outlet of
activated sludge plants A (control) and B (experimental) during
the experimental period.
microorganisms. Filamentous microorganisms are believed to form a ‘‘backbone’’ of activated sludge flocs,
on which floc-forming bacteria are fixed by means of
extracellular polymers [37]. Up to date, the effects of
Cr(VI) on filamentous microorganisms have not been
studied. Comparing the effect of other heavy metals,
Shuttleworth and UnZ [25] reported that the flocforming microorganism, Z. ramigera, was less sensitive
to metal toxicity than filamentous microorganisms. This
phenomenon is probably due to the fact that filamentous
bacteria extend trichomes outward from activated
sludge flocs, into the bulk water. In this way, they come
into contact with wastewater constituents, including
toxic compounds, more readily than microorganisms
embedded in the flocs.
Because of the lack of filamentous microorganisms,
activated sludge flocs in plant B relied solely on the
existence of extracellular microbial polymers to main-
At the start of the experiments, the microbiology of
both activated sludge systems was similar, containing
several species of stalked ciliates, free swimming ciliates
and rotifers. As it was expected, the use of synthetic
substrate lead to a gradual reduction of the microorganisms variability. Throughout the experiments, the
control system developed sludge containing certain
species of protozoa (Vorticella spp., Opercularia sp.)
and rotifers. On the other hand, addition of 1 mg l1
Cr(VI) led to the gradual disappearance of rotifers and
the increase of free-swimming ciliates in the experimental system (plant B). After deflocculation, which
started to occur at 3 mg l1 Cr(VI), free swimming
ciliates and flagellates predominated due to the sudden
availability of the food source (dispersed bacteria).
Further increase of Cr(VI) concentration to 5 mg l1
resulted in complete washout of protozoa from the
activated sludge.
The decrease in the diversity of protozoa species has
been reported in the presence of various heavy metals
[38,39]. The importance and role of the protozoa in the
purification process of activated sludge have been well
documented [29]. These organisms remove nonflocculated bacteria from wastewater, yielding at a clarified
effluent and contribute to biomass flocculation through
production of fecal pellets and mucus [17].
Concerning the effect of Cr(VI) on higher microorganims of activated sludge process, it should be
mentioned that activated sludge systems receiving
synthetic substrate, may not be as resilient to
changes in conditions as those developed on a more
diverse substrate (such as domestic wastewater).
Nevertheless, the above observations revealed the
sensitivity of different microbial species in the presence
of Cr(VI).
4. Conclusions
The experimental results presented in this study
showed that nitrifying microorganisms are more sensitive to Cr(VI) than heterotrophic microorganisms. Even
0.5 mg l1 of Cr(VI) inhibited significantly the nitrification process, while only concentrations up to 5 mg l1 of
Cr(VI) caused a slight reduction in CODdis removal.
Cr(VI) concentration equal to 1 mg l1 caused the
disappearance of rotifers from the activated sludge,
while higher concentrations affected the abundance of
protozoa that were present.
A.S. Stasinakis et al. / Water Research 37 (2003) 2140–2148
Studies on the effect of Cr(VI) on the size and
structure of activated sludge flocs showed that Cr(VI)
affected the abundance of filamentous microorganisms.
As a result of Cr(VI) addition, the experimental system
developed a sludge containing pin-point flocs resulting
in poor clarification and significant solid losses through
the secondary effluent.
Termination of Cr(VI) addition allowed partial
recovery of nitrification. Finally, shock loading with
5 mg l1 of Cr(VI) for 2 days affected the nitrification
process, while organic removal and sludge settling were
not affected.
Acknowledgements
A.S. Stasinakis would like to thank the Greek
Scholarship Foundation for financial support of this
work.
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